People often argue against homosexuality because they say it isn’t “natural”. However, if you actually go into nature, you will find some pretty crazy stuff happening. One of those crazy things is the switching of sex during the lifetime of an organism. Some species of fish are known to undergo these sex changes during their lifetime, and I thought it would be fun to tell you about them.
So, organisms that change their sex can be either 1) simultaneous hermaphrodites which produce both eggs and sperm or 2) sequential hermaphrodites, functioning as male during one part of their life and then as a female at another point in their life. For those that are sequential hermaphrodites, they can either be protandrous, where they develop first as males and then later become females, or they can be protogynous, where they develop first as females and later become males.
Ok, so now that is cleared up, time for the fun examples!! The Indo-Pacific cleaner wrasse, Labroides dimidiatus, forms harems of one large male and several females. Within the group of females, there is a hierarchy of dominance. If the dominant female is removed, the next largest female assumes the dominant female role. If the male is removed, the dominant female of the harem will start courting females and will actually develop functional testes within 2 weeks.
Another really cool example is that of the anthiine serranid, Anthias squamipinnis. This species forms large aggregations in which females greatly outnumber the males. If seven males are removed from the group, exactly seven females will change sex to replace them. If the females outnumber the males by too much, females will change to males to balance out the sex ratio again.
Perhaps the best know example is that of the clownfish group of fishes (Amphiprion spp.) These fish live in groups with two large individuals and several smaller ones. The largest fish will be a female, with the next largest being a male. The several smaller ones are all male. If the female dies, the dominant male will change to a female and then the next largest male will assume the dominant male role and grow rapidly in size.
Now, for a simultaneous hermaphrodite example. The mangrove rivuline (Rivulus marmoratus) is capable of fertilizing itself!!! The self-fertilization is all internal and results in genetically identical hermaphroditic fish. Functional males can be produced, but is often dependent on temperature and length of the day.
So, now the question arises as to why this happens. Animals often change gender when, at a given size, being one sex is more advantageous than the other. For example, small males have no problem of creating a surplus of sperm, however, smaller females produce less eggs. So, it might be more advantageous to be a female later in life when the fish is larger and can produce more eggs. However, there is a flipside to this. If you are a large male, you can monopolize many females and fertilize more of their eggs, so being a bigger male might be better. So, neither is the right answer, and clearly, both systems are utilized. For the fish that self-fertilizes itself, they live in isolated low-density populations. Self-fertilization is a means of assuring a mate.
Today’s blog post came to me as the result of an acquaintance that rides the bus. He knows I study fish and asked for a fun fish fact. The information I shared with you comes from the book The Diversity of Fishes. I had the great pleasure of meeting one of the authors, Gene Helfman, at a course I took at Friday Harbor Labs. It’s an easy to read textbook that introduces you to a lot of good info on fishes. Hope to share some more fun fish facts with you in the future!!